1. Field of the Invention
The present invention relates to mobile stations, mobile communication systems and mobile communication methods.
2. Description of the Related Art
The Radio Access Network Working Groups (RAN WGs) in the 3rd Generation Partnership Project (3GPP), an international standards body for third-generation mobile communication systems, have been formulating detailed specifications for a standard adopting the Wideband Code Divisional Multiple Access (W-CDMA) as a multiple access scheme of radio access.
Consequently, basic specifications have been completed in Release 99 so far. Also, specifications for the High Speed Downlink Packet Access (HSDPA) for improving efficiency of downlink packet-switched data transmission have been completed in Release 5 specifications. In addition, specifications for the Enhanced UpLink (EUL) for improving efficiency of uplink packet-switched data transmission are currently in the works to be included in Release 6 specifications.
In the EUL specifications, adoption of the Hybrid Automatic Repeat reQuest (HARQ) retransmission, which has been adopted in HSDPA, has been determined.
The HARQ retransmission is a scheme in which high-speed user data retransmission control is performed between a mobile station and a radio base station in the physical layer and the Medium Access Control (MAC) sublayer.
Here, the radio base station corresponds to a function (i.e., cell) for control radio communications with mobile stations located in a specific area. At least one radio base stations can be included in one radio base station apparatus.
In Release 99, the Radio Link Control (RLC) retransmission, which is performed in the RLC sublayer between a mobile station and a radio network controller, is adopted as a retransmission control scheme.
Introduction of the HARQ retransmission is expected to reduce delay caused by retransmission, improving radio transmission efficiency.
However, even when the HARQ retransmission is introduced, it is necessary to cover what the HARQ retransmission cannot handle by the RLC retransmission.
For example, in the HSDPA, which has already adopted the HARQ retransmission, control signal error in the HARQ retransmission is covered by the RLC retransmission.
In addition, the RLC retransmission caused by the control signal error is reduced to a desired probability by controlling transmission power of a High Speed Shared Control CHannel (HS-SCCH) and a threshold used for determining the presence or absence of an acknowledgement signal, in order to reduce delay caused by retransmission.
In the EUL, however, unlike in the HSDPA, diversity handover is employed. Therefore, the same control as in the HSDPA can cause frequent occurrence of the RLC retransmission. Consequently, delay caused by retransmission cannot be reduced, leading to deterioration in radio transmission efficiency.